This invention relates to a multi-layer recording medium, such as a multi-layer optical disc, having plural information recording layers in which information signals are recorded by pits. The present invention also relates to a method for producing the multi-layer recording medium.
In a disc-shaped recording medium, for example, an optical disc or a magneto-optical disc, as a package medium for music or picture information or recording apparatus for computers, the recent tendency is towards higher recording density of information signals. For realizing this high density recording, there is proposed a multi-layer optical disc having a plurality of layered information recording layers functioning as a signal recording region.
Among a variety of multi-layer optical disc, there is a double-layer optical disc having two layered information recording layers functioning as an information recording region. Such double-layer optical disc 100, shown in FIG. 1, includes a disc substrate 101 of a transparent synthetic resin material, such as polycarbonate, on one surface of which is formed a first information recording layer 101a having rows of pits, and a semi-transparent thin reflecting film 102 of a dielectric material, such as silicon nitride SiN, formed on the first information recording layer 101a. The double-layer optical disc 100 also includes an intermediate layer 103 of a transparent resin material having, on the surface thereof opposite to the recording layer 101a, a second information recording layer 103a also having rows of pits, and a reflecting film 104 of, for example, A1, formed on the second information recording layer 103a. In addition, the double-layer optical disc includes a protective layer 105 formed on the reflective film 104.
For reading out the information signals from the double-layer optical disc, the focal position of a laser light radiated from an objective lens 106 of an optical pickup device shown in FIG. 1 is changed in two steps. That is, for reading out the information signals on the first information recording layer 101a, the spot of a laser light beam L.sub.1 is located on the row of pits recorded on the first information recording layer 101a. For reading out the information signals on the second information recording layer 103a, the spot of a laser light beam L.sub.2, shown by a broken line in FIG. 1, is located on the row of pits recorded on the second information recording layer 103a. If, in such case, it is desired to prohibit interference from being produced between the information recording layers 101a, 103a in the signal read-out or reproducing operation, it is necessary to set the thickness of the transparent intermediate layer 103 between the information recording layers to a larger value on the order of 30 to 60 .mu.m, for example, 40 .mu.m.
In such double-layer optical disc 100, the intermediate layer 103 is formed by any of the following two methods. The first method is to spin-coat a UV curable resin (UV resin). The second method is to apply the UV resin dropwise into a space defined between the layers 101 and 105 by a spacer having a thickness corresponding to the thickness of the intermediate layer 103 for pressure bonding and curing the intermediate layer from both side layers.
For spin-coating the UV resin for forming the intermediate layer, since the UV resin can be applied to a thickness only of 10 .mu.m by one coating operation, the process of application followed by curing needs to be repeated e.g., four times, thus consuming much time in the production of the multi-layer optical disc. In addition, if spin-coating is repeated a number of times, the UV resin is dripped or turned round to the peripheral portion thus producing thickness variations in the intermediate layer to obstruct production of the high precision multi-layer disc.
With the second method of dripping the UV resin into the spacer and pressure bonding and curing the resin from both side layers, it is necessary to set the spacer in position, thus necessitating a time-consuming operation in the production of the multi-layer optical disc, as in the first method explained above. On the other hand, the UV resin tends to leak from the spacer thus obstructing the production of the high precision multi-layer disc.